Tone Applier explained DSCN0461

In the old long distance network there were Tone Appliers.
The idea for them was due to the fact that most busy signals in earlier times
flashed supervision. Tone Appliers were added in some tandem switches
because it was thought that should there be a call that goes through many
long distance links, possibly the sound of a busy signal could get lost in the noise. So they added the tone applier that put on the lowtone burst
on the originating end so that a clear and loud busy could be heard. Often times on the old 2600 Hz controlled long-distance microwave trunks of the early and mid-1970sm when your called party answered, quite often could be heard this burst of lowtone, even though by then, the old flashing busy signals were long gone. The tone appliers were left in.